As used herein, the term “double layer forming fabric” refers to forming fabrics comprising two sets of yarns oriented in a first direction, one set located on the paper side and the other set located on the machine side of the fabric, and which are bound together by a single set of binder yarns oriented in a transverse direction and woven as pairs. In the fabrics of this invention, the binder yarns are warp yarns.
The term “binder yarn” refers to a yarn which occupies a path in the paper side layer and which separately interlaces with a machine side layer yarn to occupy a path in the machine side layer. The corresponding term “pair of binder yarns” refers to two binder yarns which together occupy a single combined unbroken warp path in the paper side of the fabric, such that when one member of a pair passes from the paper side layer to the machine side layer, the second member of the pair exchanges position with the first member by passing from the machine side layer to the paper side layer, thus completing the weave pattern while binding the two layers together.
The term “drainage area”, expressed as a percentage of the area of the fabric weave pattern repeat, refers to the proportion of that area not occupied by the yarns, both warp and weft, used in weaving the fabric at a given substantially planar location within the fabric substantially parallel to the paper side surface and to the machine side surface of the forming fabric. The method of calculation of the drainage area at a particular location is discussed further below.
The term “float” refers to that portion of a component yarn which passes over a group of other yarns in the fabric without interweaving or interlacing with them; the associated term “float length” refers to the length of a float, expressed as a number indicating the number of yarns passed over. A float length can be expressed in terms of numbers of paper side layer or machine side layer warp or weft yarns.
The term “internal float” refers to that portion of a component yarn which passes between two sets of yarns; the associated term “internal float length” in relation to this invention refers to the length of an internal float, expressed as a number indicating the number of PS yarns passed under.
The term “inverted” means that the interweaving pattern of the pair members of a first MD yarn pair is reordered so as to be opposite to the interweaving pattern of an immediately adjacent MD yarn pair. This is effected by changing the order of insertion of the first and second members of a yarn pair in relation to an order of insertion into the overall repeating weave pattern of the first and second members of an immediately preceding pair, so that the direction of one warp yarn exchange is opposed to that occurring in the next closest exchange for the adjacent pair.
The term “machine direction”, or “MD” refers to a line parallel to the direction of travel of the forming fabric when in use on the papermaking machine. The term “cross machine direction” or “CD” refers to a direction substantially perpendicular to the machine direction within the plane of the fabric. In the fabrics of the invention, the binder warp yarns are woven in the MD.
The term “paper side layer” refers to the layer in the forming fabric onto which the stock is delivered from the head box slice. The term “machine side layer” refers to the layer in the forming fabric in contact with the support means in the papermaking machine. Thus each of these layers has a paper side (“PS”) surface and a machine side (“MS”) surface. In the double layer fabrics of the invention, the machine side surface of the paper side layer is adjacent to the paper side surface of the machine side layer.
The term “segment” refers to a portion of the single path occupied by a specific binder yarn in one repeat of the overall weave pattern, and the associated term “segment length” refers to the length of a particular segment, and is expressed as the number of paper side layer yarns with which a member of a pair of binder yarns interweaves within the segment.
In the papermaking process, a very dilute slurry of about 99% water and 1% papermaking fibers is ejected at a very high speed and precision from a headbox slice onto a moving forming fabric, which is used to retain and support the papermaking fibres in the stock, to allow water to drain from the stock so that an embryonic fibrous web may form and to convey that web to subsequent areas of the papermaking machine.
To overcome the known disadvantages of single layer fabrics for use in the forming section, double layer forming fabrics have been developed which consist essentially of two layers: these are a paper side layer which provides the surface on which an incipient paper web is formed, and a machine side layer which provides the surface that is in contact with the static supporting surfaces of the paper making machine. As noted above, within the overall forming fabric weave pattern, either warp yarns or weft yarns can be used as binder yarns which serve to hold the layers of the double layer fabric together and may contribute to the structure of one of the layers, but in the fabrics of the invention the binder yarns are warp yarns.
It then follows that although the layers are bound together by the weaving process into a single fabric with a single overall repeating weave pattern, each of the layers is often constructed quite differently in terms of yarn sizes, yarn cross sectional shapes, yarn count (in terms of numbers of yarns per unit length), yarn fill (expressed as a percentage of the amount of yarns and their size relative to the total space available to accommodate them) and the thermoplastic polymer used in the yarns. Thus, at least the water handling capabilities, the wear resistance capabilities, and the strength capabilities of each layer, when considered separately, can be and commonly are quite different.
Modern forming fabrics are woven so as to provide a paper side layer which imparts, amongst other things, a minimum of fabric mark to, and provides adequate drainage of liquid from, the incipient paper web. The paper side layer should also provide maximum support for the fibres and other papermaking solids in the paper slurry. The machine side layer should be tough and durable, and provide a measure of dimensional stability to the forming fabric so as to minimize fabric stretching and narrowing, or other distortions.
The role of forming fabrics is particularly significant in relation to their specific contribution to sheet properties, in that excessive drainage will adversely affect good sheet formation. In particular, any irregularities in drainage, including those resulting from variations in the main body of the fabric or at the seaming area, will generally result in marking, i.e. visible imperfections in the appearance of the eventual finished sheet; or in inconsistencies which affect tear strength and good printing quality.